Sealing the surface of waste dumps is very important in terms of environmental protection. The long-term physical and chemical interactions of the dump with the terrain in which the dump has been set up are critical. With regard to danger to the environment (hazard potential), 5 different dump classes have been defined, in the following order:
Dump class 5=Special wastes PA0 Dump class 4=Commercial and industrial wastes PA0 Dump class 3=Domestic wastes PA0 Dump class 2=Building rubble PA0 Dump class 1=Excavated earth.
In this sequence, the hazard potential decreases from top to bottom; that is to say, the special wastes are attributed the highest hazard potential and the excavated earth the lowest hazard potential. To minimise migration and washing of pollutants out of dumps, a number of regulations on sealing the surface of dumps have recently been defined. Thus, for example for sealing the surface of a dump for special wastes (dump class 5), the following covering layers are prescribed from the inside outward:
A compensating layer of homogeneous, non-binding material, which is at least 0.5 m thick, above the special waste;
over this, where there is a possibility of gas formation, a continuous filtration-stable drainage layer at least 0.3 m thick for gas drainage;
above this a mineral seal which is at least 0.5 m thick and has a permeability k.ltoreq.5.times.10.sup.-10 m/s;
on top of this (as a combination seal with the mineral seal), an HDPE plastic sheet at least 2 mm thick;
above this, a 0.3 m thick mineral drainage layer;
and, as the uppermost layer, a 1.0 m to 1.5 m thick protective and vegetation layer such as soil which can be cultivated and planted.
For dumps for domestic waste (dump class 3), a surface seal comprising the following sequence of layers is specified:
Above the waste of dump class 3, a compensating layer;
above this, a gas drain which is at least 0.2 m thick and may be provided with geotextile;
above this, a 0.5 m thick mineral seal having a permeability of k.ltoreq.5.times.10.sup.-10 m/s;
above this, possibly a plastic sealing sheet;
above this, a drainage layer at least 0.2 m thick;
and finally once again a protective and recultivation layer.
According to a guideline for building rubble dumps, the following requirements are set for sealing the surface of dumps for building rubble and wastes resembling building rubble (dump class 2):
The thickness of the surface seal should not be less than 50 cm and that of the water-removal layer (drainage layer) should not be less than 30 cm. The permeability of the sealing layer must be k.ltoreq.1.times.10.sup.-8 m/s and that of the drainage layer must be k.ltoreq.1.times.10.sup.-4 m/s.
For dumps for excavated earths (dump class 1), the same draft guideline requires no surface seals.
The surface-sealing systems for dump classes 3 to 5 according to the present state-of-the-art, which comprise a combination seal of a mineral seal with a plastic sheet lying directly on top, have the following disadvantages:
The plastic sealing sheet has only limited aging resistance, only limited resistance to microorganisms and only limited stability through root penetration and attack by rodents;
particularly on slopes, there are still unsolved problems with regard to the danger of the slipping of the soil layers on the plastic sealing sheet;
because of the small root penetration depth (&lt;2 m) unrestricted planting is not possible. Uncontrolled growth even by deep-rooted plants (for example trees) through natural seeding is to be prevented;
there is no high stability against wind, particularly in the event of incorrect growth.
In these circumstances, it can be seen that the long-term stability and erosion resistance of the known surfacesealing systems are unsatisfactory.